How can I do pagination with Bluebird Promises?

Question

I have something like

new Promise (resolve, reject) ->
  trader.getTrades limit, skip, (err, trades) ->
    return reject err if err

    resolve trades
.each (trade) ->
  doStuff trade

limit is set to some arbitrary number, say 10 and skip starts at 0. I want to keep increasing skip until there are no more trades.

The doStuff is a function I'm using to process each trade.

This works the first time, but I want to get more trades in a paginated fashion. Specifically, I want to run trader.getTrades with a higher skip until trades.length is 0

Answer 1

You should be able to use promisify()/promisifyAll() to convert trader.getTrades() to an async version that returns a promise. Then, something like this should work well:

function getAllTrades(limit, offset, query) {

    var allTrades = [];

    function getTrades(limit, offset, query){
        return trader.getTradesAsync(limit, offset, query)
            .each(function(trade) {
                allTrades.push(trade)
                // or, doStuff(trade), etc.
            })
            .then(function(trades) {
                if (trades.length === limit) {
                    offset += limit;
                    return getTrades(limit, offset, query);
                } else {
                    return allTrades;
                }
            })
            .catch(function(e) {
                console.log(e.stack);
            })
    }

    return getTrades(limit, offset, query)
}

If you knew the total # of trades in advance you could use a different strategy with .map and {concurrency: N} to get N pages of trades at once.